1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission for an automobile and, more particularly, to an automatic transmission shift control system for an automobile which reduces shift shock.
2. Description of Related Art
Automatic transmissions for automotive vehicles typically include first or primary transmission means, selectively shiftable into first to third speed gears, and second or secondary transmission means, selectively shiftable into low and high speed gears. The automatic transmissions are each shifted into various forward speeds by combinations of speed gears of the primary and secondary transmission means.
In such an automatic transmission, an automatic shift may occur in which the primary transmission means shifts to a high speed gear while the secondary transmission means shifts to a lower speed gear. Alternatively, the automatic shift may involve a shift of the primary transmission means to a low speed gear while the secondary transmission means shifts to a higher speed gear. When a shift in which the primary and secondary transmission means are "inverse" in shift direction relative to each other occurs, the shift typically causes what is called "shift shock."
In a specific situation, an automatic transmission may shift from one speed to another speed by skipping over an intermediate speed. In such a situation, a transitional shift to the intermediate speed may occur. For example, a down-shift from a fourth speed to a second speed may be performed by skipping over an intermediate speed, namely, a third speed. For such a down-shift, the primary transmission means shifts down from a third speed gear to a first speed gear, skipping a second speed gear, while the secondary transmission shifts up from a lower speed gear to a higher speed gear. If an up-shift of the secondary transmission means is completed while the second speed gear is transitionally created by the primary transmission means, the automatic transmission will temporarily complete an up-shift to the third speed during the fourth to second (4-2) down-shift. If this occurs, a steep decline in transmitting torque in the automatic transmission is produced. The steep torque decline produces a shift shock which is physically felt by the driver. Such a shift shock, due to a steep torque decline, is induced during any up-shift which skips over an intermediate speed as well as a down-shift which skips over an intermediate speed.